The present invention relates to a process for coating a substrate. More particularly, the invention relates to coating a substrate with a zinc oxide-containing material, preferably an electrically conductive zinc oxide-containing material.
An application where substrates with coatings, e.g., electrically conductive coatings, find particular usefulness is in the promotion of chemical reactions, e.g., gas/liquid phase reactions, electro catalytic reactions, photo catalytic reactions, redox reactions, etc. As an example of a type of reaction system, a catalytic, e.g., metallic, component is contacted with the material to be reacted, e.g., an aliphatic alcohol is passed through or near to the catalytic component to enhance the chemical reaction, e.g., the dehydrogenation to a ketone. In addition, using a substrate for the catalytic component which is coated with an electrically conductive material is highly advantageous for electro and photo electro catalysis since a field/current can be effectively and efficiently provided to or near the catalytic component for electron transfer reactions. Many types of chemical reactions can be advantageously promoted using such coated substrates. Zinc-oxide containing coatings on substrates may promote electron transfer whether or not the chemical reaction is conducted in the presence of a electro, photo electro current or field. In addition, zinc oxide coated substrates and sintered zinc dioxides are useful as gas sensors, optically transparent conductive films and acoustic biezo transducer type devices and articles. One or more other components, e.g., metal components, are often included in certain of these applications.
In many of the above-noted applications it would be advantageous to have an electrically, electronically conductive; electro mechanical zinc oxide which is substantially uniform, has high electronic conductivity, and has good chemical properties, e.g., morphology, stability, etc.
A number of techniques may be employed to provide conductive zinc oxide coatings on substrates. For example, the chemical vapor deposition (CVD) process may be employed. This process comprises contacting a substrate with a vaporous composition comprising a zinc component and a dopant-containing material and contacting the contacted substrate with an oxygen-containing vaporous medium at conditions effective to form the doped zinc oxide coating on the substrate. Conventionally, the CVD process occurs simultaneously at high temperatures at very short contact times so that zinc oxide is initially deposited on the substrate. However zinc oxide can form off the substrate resulting in a low reagent capture rate. The CVD process is well known in the art for coating a single flat surface which is maintained in a fixed position during the above-noted contacting steps. The conventional CVD process is an example of a "line-of-sight" process or a "two dimensional" process in which the zinc oxide is formed only on that portion of the substrate directly in the path of the zinc source as zinc oxide is formed on the substrate. Portions of the substrate, particularly internal surfaces, which are shielded from the zinc oxide being formed, e.g., such as pores which extend inwardly from the external surface and substrate layers which are internal at least partially shielded from the depositing zinc oxide source by one or more other layers or surfaces closer to the external substrate surface being coated, do not get uniformly coated, if at all, in a "line-of-sight" process. Such shielded substrate portions either are not being contacted by the zinc source during line-of-sight processing or are being contacted, if at all, not uniformly by the zinc source during line-of-sight processing. A particular problem with "line-of-sight" processes is the need to maintain a fixed distance between the zinc source and the substrate. Otherwise, zinc dioxide can be deposited or formed off the substrate and lost, with a corresponding loss in process and reagent efficiency.
One of the preferred substrates for use in electro mechanical devices, such as transducers, are inorganic substrate, in particular flakes, spheres, fibers and other type particles. Although the CVD process is useful for coating a single flat surface, for the reasons noted above this process tends to produce non-uniform and/or discontinuous coatings on non-flat, non-equidistant surfaces. Such non uniformities and/or discontinuities are detrimental to the electrical, optical, acoustic and chemical properties of the coated substrate. A new process, e.g., a "non-line-of-sight" or "three dimensional" process, useful for coating such substrates would be advantageous. As used herein, a "non-line-of-sight" or "three dimensional" process is a process which coats surfaces of a substrate with zinc oxide which surfaces would not be directly exposed to vaporous or other zinc oxide-forming compounds being deposited on the external surface of the substrate during the first contacting step. In other words, a "three dimensional" process coats coatable substrate surfaces which are at least partially shielded by other portions of the substrate which are closer to the external surface of the substrate and/or which are further from the zinc oxide forming source during processing, e.g., the internal and/or opposite side surfaces of a glass or ceramic fiber or spheres, or flakes or other shapes or surfaces.
Although a substantial amount of work has been done, there continues to be a need for a new method for coating substrates, particularly three dimensional substrates with doped zinc oxide. The prior art processes described below follow conventional processing techniques such as by sintering of zinc oxide and or the instantaneous conversion to zinc oxide by spray pyrolyis.
For example, in "Formulation of electrically conductive, thermal-control coatings" by Shai, Michal C. (Goddard Space Flight Cent., NASA, Greenbelt, Md. USA) NASA Tech Memo. 1977, NASA TM-X-73537, three materials for electrical conductor thermal coatings of the International Sun Earth Explorer satellite were studied. Combinations of ZnO, Al.sub.2 O.sub.3, Co.sub.3 O.sub.4, and Na or K silicate were used. The coatings were fabricated by stirring oxide aqueous slurries for 5 hours, heating 48 hours at 90.degree. C., 72 hours at 110.degree. C., and firing at 1175.degree. C. for 15 minutes. Coatings were satisfactory for these conventionally prepared ZnO materials.
In the "Study of the Hall effect in the chemisorption of molecular oxygen on zinc oxide," Garcia de la Banda, J. F.; Canosa, B.; Fernandez, M.; Pajarea J. Zh. Fiz. Khim. 1979, 53(11), 2895-900 (Russ), the effect of thermal pretreatment (600.degree.-900.degree. C. in air) and doping (&lt;0.1 weight% Ga) was studied on the thickness of a subsurface electron depleted layer which forms due to chemisorption of O on polycryaline. ZnO. The current carriers decreased with the increase of temperature during the preliminary thermal treatment owing to the atmosphere 0 penetration and reaction with Zn atoms. The limiting layer thickness found was zero when treated at 900.degree. C. and it reaches 800-1700 Ao when treated at 800.degree.-700.degree. C. The samples treated at 600.degree. C. lose 99% current carriers owing to 0 chemisorption.
In "Electroconductive zinc oxide" Muramoto Makoto; Ishida, Nobuyuki; Matsushita, Fujio Japan Kokai Tokyo Kobe 80 10,478 (CL C01G9/02), 24 Jan 1980, Application. 78/84,812, electroconductive ZnO powder useful in preparing coatings for electrostatic recording, electrorecording, electrical discharge recording, and facsimile recording were obtained by sintering a ZnO-Al.sub.2 O.sub.3 (100:0.05-2.5 mol ratio) mixture at 800.degree.-1000.degree. C. in the presence of C and cooling to .ltoreq.20 nm. Thus, ZnO and Al.sub.2 O.sub.3 were mixed in a 100:0.5 mol ratio, the mixture (30 parts) was placed on a layer of charcoal pellets (8 parts) in a crucible, sintered at 1000.degree. C. for 1 h, and cooled in a nitrogen stream to give a powder having a volume resistivity of 2.7.times.10.sup.3 ohm-cm.
In "Manufacture of electrically conducting fine zinc oxide." Nihon Kagaku Sangyo Co., Ltd., Japan Kokai Tokyo Kobe 81 69.266 (CL C04B35/00), 10 Jun 1981, Application 79/141,989, 06 Nov 1979; an electrically conducting fine powder ZnO useful for electrostatic recording paper is prepared from an aqueous solution containing a Zn salt and a compound selected from Al.sub.2 O.sub.3, Sn oxide, Co oxide, Cr oxide, Ti oxide, Ge oxide, Ga oxide, and In oxide by neutralizing the materials, presintering, and sintering in gas at 600.degree.-1000.degree. C. Thus, ZnCl.sub.2 170 and AlCl.sub.3 13.5 g were dissolved in 1 liter of H.sub.2 O and 14% NH.sub.4 OH was slowly added to adjust the pH to 8.12. The material was filtered, dried, heated, at 450.degree. C. for 1 hour and sintered in a nitrogen gas at 800.degree. C. for 60 minutes to obtain 94 g of an electrically conductive ZnO having an intrinsic resistivity of 6.7.times.10.
In "Preparation of zinc oxide conductors." Shiromizu Kagaku Kogyo K. K. Japan Koka] Tokyo Kobe JP 83,161.923 [83,161,923] (CL CO1G9/00), 26 Sep 1983, Application 82/41,947, 17 Mar 1982; ZnO (100 parts) and a compound (0.5-20 parts) selected from Al.sub.2 O.sub.3, Ti oxide, and Sn oxide (or their compounds which give oxides upon thermal decomposition) are treated in an aqueous medium in the presence of compound (5-100 parts) selected from (NH.sub.4).sub.2 CO.sub.2, NH.sub.4 HCO.sub.3, NH.sub.4 Cl, NH.sub.4 NO.sub.2 (NH.sub.4).sub.2 SO.sub.4, and urea. The treated mixture is fired in a reducing atmosphere to give conductive ZnO. ZnO (100 g) was dispersed in H.sub.2 O, then a mixture of (NH.sub.4).sub.2 CO.sub.2 (130 g) and Al.sub.2 (SO.sub.4).sub.2 (5 g as octahydrate) solution was added and the dispersion heated to 60.degree. C. The mixture was then filtered and the filter cake fired at 800.degree. to give an Al-doped ZnO having good conductivity. The conductor was useful in preparing electrostatic imaging sheets.
In "Thin-film surface-acoustic-wave devices." Mitsuyu, Tsuneo; Ohji, Kenzo; Ono, Shusuke; Yamazaki, Osamu; Wasa, Kiyotaka (Mater. Res. Lab., Matsushita Electr. Ind. Co., Osaka, Japan), Natl. Tech. Rep. ((Matsushita Electr. Ind. Co., Osaka) 1976, 22(6), 905-23 (Japan), highly oriented radio-frequency sputtered films of ZnO and Bi.sub.12 PbO.sub.19 were prepared. A hemispherical electrode configuration was very effective in orienting ZnO in the c axis on a glass plate. The obtained electromechanical coupling factor was up to 88% of the bulk single-crystal value.
In "Antistatic plastic films." Fuji Photo Film Co., Ltd. Japan Kokai Tokyo Kobe JP 58 63,726[83 63,726] (CL C08J7/04), 15 Apr 1983, Application 81/161,294, 09 Oct 1981; antistatic coatings on plastic films are formed in such a manner that the surface of the antistatic layers containing .gtoreq.1 c/c powder selected from oxides of Zn, Ti, Sn, In, Si, Mo and W, and their composite oxides exhibits a citron-skin like structure. Such a surface structure can be formed by using a mixture of solvents having different evaporation rates. The antistatically treated films are especially useful as photographic film supports. Thus, a conductive SnO.sub.2 --Sb.sub.2 O.sub.3 composite and styrene-maleic anhydride copolymer were mixed in MeOH, Me.sub.2 CO, and ethyl cellosolve and the mixture was coated on poly(ethylene terephthalate) film precoated with a cellulose acetate and polyester, to give an electrically conductive layer with a citron skin-like structure.
In "Electrostatic recording materials." Oji Paper Co., Ltd. Japan Kokai Tokyo Kobe JP 58 04,132 [83 04,152] (Cl.G03G5 08) Jan 1983. Application 81,101,305, 01 Jul 1981; electrostatic recording materials having a conductor layer made of a binder and conductive ZnO prepared by calcination of a mixture of ZnO, Al.sub.2 O.sub.3 or Al compound which can be converted to Al.sub.2 O.sub.3 by calcination, and S at 600.degree.-900.degree. C. The ZnO, Al.sub.2 O.sub.3, and S 1.0 mol/100 mol ZnO were calcined at 800.degree. C. to give conductive ZnO powder having good whiteness. The ZnO powder was dispersed in a polyvinyl alcohol solution coated on a paper support, having an acrylic resin and CaCO.sub.3 to form a dielectrical layer on the conductor layer. The resultant electrostatic recording paper had good whiteness and recording characteristics.
In "Optical and electronic properties of zinc oxide films prepared by spray pyrolysis." Major S.; Banerjee, A; Chopra, K. I. (Cent. Energy Stud., Indian Inst. Technol., New Delhi, 110016 India.) Thin Solid Films 1985, 125(0 2). 179-85 the optical properties of transparent conducting ZnO films prepared by spray pyrolysis were studied in the UV visible and IR regions. The specular reflectance and transmittance data were used to determine the optical constants which correlate well with the data on single-crystal ZnO in the visible region. The films doped with 3 weight % indium exhibit thermal stability up to 650K in vacuum and up to 450K in O ambients. The changes in the electronic properties of pure ZnO films on annealing in O and vacuum were attributed to chemisorption and desorption of O at grain boundaries.